Blue-green algae (cyanobacteria) blooms in Pets

Toxic blue-green algae can kill a dog within hours of swimming. Learn how blooms look, the liver and nerve signs to watch for, and why rinsing and fast vet care matter.

What It Is

Blue-green algae are cyanobacteria that can grow into harmful blooms in warm, still, nutrient-rich water. Some produce toxins: microcystins damage the liver, while anatoxins attack the nervous system and can kill within minutes to hours. You cannot tell whether a bloom is toxic by looking at it.

Where Pets Come Across It

Ponds, lakes, reservoirs and slow rivers, especially in late summer. Blooms look like green paint, pea soup, scum or mats, and may wash up on shorelines. Dogs drink the water, swim in it, lick their coats afterwards or eat dried mats on the shore.

Which Animals Are Affected

Dogs are most often affected because they swim and lick their fur. Livestock, wildlife and people are also at risk.

Published Toxic Dose

No owner-usable mg/kg figure exists: toxin concentrations in a bloom vary from none to lethal and change by the hour. The Merck Veterinary Manual notes that clinical signs may start within minutes to days of exposure. The calculator therefore offers no dose grading for algae.

Signs and How Soon They Appear

Nerve toxins: drooling, tremors, stiffness, seizures, difficulty breathing and collapse within minutes to a few hours. Liver toxins: vomiting, diarrhoea, weakness, pale gums and jaundice over hours to a few days.

What to Do Right Now

Rinse your dog thoroughly with clean water (wear gloves) to stop it licking toxins off its coat, and go to a vet immediately, especially if any signs have started. Tell the vet where and when the exposure happened. Do not induce vomiting. Report the bloom to your local environmental agency.

Frequently Asked Questions

Sources and References

• Merck Veterinary Manual: Algal Poisoning of Animals
• Pet Poison Helpline: Blue-Green Algae

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